1. Field of the Invention
This invention relates generally to oilfield operations and more particularly to the downhole apparatus utilizing fiber optic sensors and use of same in monitoring the condition of downhole equipment, monitoring certain geological conditions, reservoir monitoring and remedial operations.
2. Background of the Art
A variety of techniques have been utilized for monitoring wellbores during completion and production of wellbores, reservoir conditions, estimating quantities of hydrocarbons (oil and gas), operating downhole devices in the wellbores, and determining the physical condition of the wellbore and downhole devices.
Reservoir monitoring typically involves determining certain downhole parameters in producing wellbores at various locations in one or more producing wellbores in a field, typically over extended time periods. Wireline tools are most commonly utilized to obtain such measurements, which involves transporting the wireline tools to the wellsite, conveying the tools into the wellbores, shutting down the production and making measurements over extended periods of time and processing the resultant data at the surface. Seismic methods wherein a plurality of sensors are placed on the earth""s surface and a source placed at the surface or downhole are utilized to provide maps of subsurface structure. Such information is used to update prior seismic maps to monitor the reservoir or field conditions. Updating existing 3-D seismic maps over time is referred to in industry as xe2x80x9c4-D Seismicxe2x80x9d. The above described methods are very expensive. The wireline methods are utilized at relatively large time intervals, thereby not providing continuous information about the wellbore condition or that of the surrounding formations.
Placement of permanent sensors in the wellbore, such as temperature sensors, pressure sensors, accelerometers and hydrophones has been proposed to obtain continuous wellbore and formation information. A separate sensor is utilized for each type of parameter to be determined. To obtain such measurements from the entire useful segments of each wellbore, which may have multi-lateral wellbores, requires using a large number of sensors, which requires a large amount of power, data acquisition equipment and relatively large space in the wellbore: this may be impractical or prohibitively expensive.
Once the information has been obtained, it is desirable to manipulate downhole devices such as completion and production strings. Prior art methods for performing such functions rely on the use of electrically operated devices with signals for their operation communicated through electrical cables. Because of the harsh operating conditions downhole, electrical cables are subject to degradation. In addition, due to long electrical path lengths for downhole devices, cable resistance becomes significant unless large cables are used. This is difficult to do within the limited space available in production strings. In addition, due to the high resistance, power requirements also become large.
One particular arrangement in which operation of numerous downhole devices becomes necessary is in secondary recovery. Injection wells have, of course, been employed for many years in order to flush residual oil in a formation toward a production well and increase yield from the area. A common injection scenario is to pump steam down an injection well and into the formation which functions both to heat the oil in the formation and force its movement through the practice of steam flooding. In some cases, heating is not necessary as the residual oil is in a flowable form, however in some situations the oil is in such a viscous form that it requires heating in order to flow. Thus, by using steam one accomplishes both objectives of the injection well: 1) to force residual oil toward the production well and 2) to heat any highly viscous oil deposits in order mobilize such oil to flow ahead of the flood front toward the production well. As is well known to the art, one of the most common drawbacks of employing the method above noted with respect to injection wells is an occurrence commonly identified as xe2x80x9cbreakthroughxe2x80x9d. Breakthrough occurs when a portion of the flood front reaches the production well. As happens the flood water remaining in the reservoir will generally tend to travel the path of least resistance and will follow the breakthrough channel to the production well. At this point, movement of the viscous oil ends. Precisely when and where the breakthrough will occur depends upon water/oil mobility ratio, the lithology, the porosity and permeability of the formation as well as the depth thereof. Moreover, other geologic conditions such as faults and unconformities also affect the in-situ sweep efficiency.
While careful examination of the formation by skilled geologists can yield a reasonable understanding of the characteristics thereof and therefore deduce a plausible scenario of the way the flood front will move, it has not heretofore been known to monitor precisely the location of the flood front as a whole or as individual sections thereof. By so monitoring the flood front, it is possible to direct greater or lesser flow to different areas in the reservoir, as desired, by adjustment of the volume and location of both injection and production, hence controlling overall sweep efficiency.. By careful control of the flood front, it can be maintained in a controlled, non fingered profile. By avoiding premature breakthrough the flooding operation is effective for more of the total formation volume, and thus efficiency in the production of oil is improved.
In production wells, chemicals are often injected downhole to treat the producing fluids. However, it can be difficult to monitor and control such chemical injection in real time. Similarly, chemicals are typically used at the surface to treat the produced hydrocarbons (i.e., to break down emulsions) and to inhibit corrosion. However, it can be difficult to monitor and control such treatment in real time.
The present invention addresses the above-described deficiencies of the prior art and provides apparatus and methods which utilize sensors (such as fiber optic sensors), wherein each sensor can provide information about more than one parameter to perform a variety of functions. The sensors are used to measure parameters related to the chemical introduction in real time so that the chemical treatment system can be accurately monitored and controlled.
The present invention addresses the above-described deficiencies of prior art and provides apparatus and methods which utilize fiber optic sensors, wherein each sensor can provide information about more than one parameter to perform a variety of functions. The sensors may be placed along any length of the wellbore. Sensor segments, each containing one or more sensors, may be coupled to form an active section that may be disposed in the casing for continuous monitoring of the wellbore. Sensors may be distributed in a wellbore or multiple wellbores for determining parameters of interest. Hermetically sealed optical fibers coated with high temperature resistant materials are commercially available. Single or multi-mode sensors can be fabricated along the length of such optical fibers. Such sensors include temperature, pressure and vibration sensors. Such sensors can withstand high temperatures in excess of 250 degrees Celsius for extended time periods and thus have been found to be useful in wellbore applications. An optical fiber is a special case of an optical waveguide and in most applications, other types of optical waveguides, including those containing a fluid, can usually be substituted for optical fiber.
The present invention provides certain completion and production strings that utilize fiber optical waveguide based sensors and devices. The invention also provides a method of generating electrical power downhole, utilizing light cells installed in the wellbore.
This invention uses fiber optic sensors to make measurements of downhole conditions in a producing borehole. The measurements include temperature and pressure measurements; flow measurements related to the presence of solids and of corrosion, scale and paraffin buildup; measurements of fluid levels; displacement; vibration; rotation; acceleration; velocity; chemical species; radiation; pH values; humidity; density; and of electromagnetic and acoustic wavefields. These measurements are used for activating a hydraulically-operated device downhole and deploying a fiber optic sensor line utilizing a common fluid conduit. A return hydraulic conduit is placed along the length of a completion string. The hydraulic conduit is coupled to the hydraulically-operated device in a manner such that when fluid under pressure is supplied to the conduit, it would actuate the device. The string is placed or conveyed in the wellbore. Fiber optic cable carrying a number of sensors is forced into one end of the conduit until it returns at the surface at the other end. Light source and signal processing equipment is installed at the surface. The fluid is supplied under sufficient pressure to activate the device when desired. The hydraulically-operated device may be a packer, choke, sliding sleeve, perforating device, flow control valve, completion device, an anchor or any other device. The fiber optic sensors carried by the cable may include pressure sensors, temperature sensors, vibration sensors, and flow measurement sensors.
This invention also provides a method of controlling production from a wellbore. A production string carrying an electrical submersible pump is preferably made at the surface. An optical fiber carrying a plurality of fiber optic sensors is placed along a high voltage line that supplies power to the pump for taking measurements along the wellbore length. In one configuration, a portion of the fiber carrying selected sensors is deployed below the pump. Such sensors may include a temperature sensor, a pressure sensor and a flow rate measurement sensor. These sensors effectively replace the instrumentation package usually installed for the pump.
In an application to control of injection wells, the invention provides significantly more information to well operators thus enhancing oil recovery to a degree not heretofore known. This is accomplished by providing real time information about the formation itself and the flood front by providing permanent downhole sensors capable of sensing changes in the swept and unswept formation and/or the progression of the flood front. Preferably a plurality of sensors would be employed to provide information about discrete portions of strata surrounding the injection well. This provides a more detailed data set regarding the well(s) and surrounding conditions. The sensors are, preferably, connected to a processor either downhole or at the surface for processing of information. Moreover, in a preferred embodiment the sensors are connected to computer processors which are also connected to sensors in a production well (which are similar to those disclosed in U.S. Pat. No. 5,597,042 which is fully incorporated herein by reference) to allow the production well to xe2x80x9ctalkxe2x80x9d directly to the related injection well(s) to provide an extremely efficient real time operation. Sensors employed will be to sense temperature, pressure, flow rate, electrical and acoustic conductivity, density and to detect various light transmission and reflection phenomena. All of these sensor types are available commercially in various ranges and sensitivities which are selectable by one of ordinary skill in the art depending upon particular conditions known to exist in a particular well operation. Specific pressure measurements will also include pressure(s) at the exit valve(s) down the injection well and at the pump which may be located downhole or at the surface. Measuring said pressure at key locations such as at the outlet, upstream of the valve(s) near the pump will provide information about the speed, volume, direction, etc. at/in which the waterflood front (or other fluid) is moving. Large differences in the pressure from higher to lower over a short period of time could indicate a breakthrough. Conversely, pressure from lower to higher over short periods of time could indicate that the flood front had hit a barrier. These conditions are, of course, familiar to one of skill in the art but heretofore far less would have been known since no workable system for measuring the parameters existed. Therefore the present invention since it increases knowledge, increases productivity.
Referring now to the measurement of density as noted above, the present invention uses fluid densities to monitor the flood front from the trailing end. As will be appreciated from the detailed discussion herein, the interface between the flood front and the hydrocarbon fluid provides an acoustic barrier from which a signal can be reflected. Thus by generating acoustic signals and mapping the reflection, the profile of the front is generated in 4D i.e., three dimensions over time.
The distributed sensors of this invention find particular utility in the monitoring and control of various chemicals which are injected into the well. Such chemicals are needed downhole to address a large number of known problems such as for scale inhibition and various pretreatments of the fluid being produced. In accordance with the present invention, a chemical injection monitoring and control system includes the placement of one or more sensors downhole in the producing zone for measuring the chemical properties of the produced fluid as well as for measuring other downhole parameters of interest. These sensors are preferably fiber optic based and are formed from a sol gel matrix and provide a high temperature, reliable and relatively inexpensive indicator of the desired chemical parameter. The downhole chemical sensors may be associated with a network of distributed fiber optic sensors positioned along the wellbore for measuring pressure, temperature and/or flow. Surface and/or downhole controllers receive input from the several downhole sensors, and in response thereto, control the injection of chemicals into the borehole.
In still another feature of this invention, parameters related to the chemical being used for surface treatments are measured in real time and on-line, and these measured parameters are used to control the dosage of chemicals into the surface treatment system.
Another aspect of the present invention provides a fiber optic device (light actuated transducer) for generating mechanical energy and methods of using such energy at the well site. The device contains a fluid that rapidly expands in an enclosure upon the application of optical energy. The expansion of the fluid moves a piston in the enclosure. The fluid contracts and the piston is pushed back to its original position by a force device such as spring. The process is then repeated to generate reciprocating motion of a member attached to the piston. The device is like an internal combustion engine wherein the fuel is a fluid in a sealed chamber that expands rapidly when high energy light such as laser energy is applied to the fluid. The energy generated by the optical device is utilized to operate a device in the wellbore. The downhole device may be any suitable device, including a valve, fluid control device, packer, sliding sleeve, safety valve, and an anchor. The motion energy generated by the fiber optic devices may be used to operate a generator to generate electrical power downhole which power is then utilized to charge batteries downhole or to directly operate a downhole device and/or to provide power to sensors in the wellbore. A plurality of such fiber optic devices may be utilized to increase the energy generated. The devices may also be used as a pump to control the supply of fluids and chemicals in the wellbore.
Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.